IPostclsia 343 



and its rings. Usually the concretions present 

 the same hardness from circumference to center, 

 but the large ones have a softer central portion. 

 When worn off by the waves and weathering, 

 they form hollows upon the surface and become 

 tide-pools, later developing into many and diver- 

 sified forms. An interesting concretion partially 

 buried in a vertical rock wall is seen in front of 

 the Station buildings. Its shape suggests that 

 it was once somewhat spherical but subsequently 

 by a reduction in thickness of the sandstone for- 

 mation as a whole through dissolving and re- 

 moving a portion of its calcium carbonate, the 

 concretion had become considerably flattened 

 (Plate XXXIII). 



The question arises: How are these con- 

 cretions formed? They consist of sand grains 

 aggregated by a cement of calcium carbonate. 

 This can be shown by dropping a fragment of 

 fresh material into a test tube containing dilute 

 chlorhydric acid. After a violent effervescence 

 the sand grains lie in a loose mass in the bottom. 

 Often in breaking a fresh concretion and holding 

 the fractured surface in the light, a reflection 



